S17-1 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation SECTION 17 ENFORCED MOTION LARGE MASS METHOD

S17-2 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation

S17-3 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION n Used to analyze constrained structures with base input acceleration, displacement, and velocity. n Common examples are earthquakes (for transient analysis) and swept-sine shaker test simulation (for frequency response analysis). n Since Version 2001 of MSC.Nastran it is possible to specify enforced motion using partitioned equations. n Prior to Version 2001 of MSC.Nastran, a technique called the large mass method was most commonly used to apply enforced motion. This is described in this section, as it can be a useful technique.

S17-4 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation USING LARGE MASS METHOD IN TRANSIENT RESPONSE n Large Mass Method to apply enforced accelerations. n In this method, apply forces {P(t)} to the structure rather than accelerations. If enforced motion is selected, it is assumed that the user is imposing the motion on a large mass. Therefore, the force to move the large mass is proportional to acceleration.

S17-5 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation USING LARGE MASS METHOD IN TRANSIENT RESPONSE (Cont.) n For enforced acceleration n Larger mass should be ~10 6 times the structure mass u If the Large Mass is too small then a coupling may occur between the Large Mass and structural mass. u If the Large Mass is too big, numerical conditioning problems may occur. Large Mass m Rigid Body Structure of Interest (M L = large Mass and Structural Mass, M, is not significant.) M=~10 6 m

S17-6 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation USING LARGE MASS METHOD IN TRANSIENT RESPONSE (Cont.) n Replace the constraints with connections to the Large Mass using an RBE2 and CONM2 method. CONM2 RBE2 Structure Connected to ground n It is vital that the RBE2 connections model the same DOFs as the original constraints, otherwise the boundary conditions and hence the response, will change.

S17-7 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation 1000 lbs mass n Please carry out Workshop 7, which uses the Direct Transient Method, to determine the response due to a unit acceleration sine pulse of 250 Hz applied at the base in the Z-direction. A large mass of 1000 lbm is connected to the base via an RBE2. n Please do not hesitate to ask your tutors advice. WORKSHOP 7 DIRECT TRANSIENT RESPONSE WITH ENFORCED ACCELERATION

S17-8 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION IN FREQUENCY RESPONSE n As in transient response, MSC.NASTRAN can directly specify acceleration, velocity, or displacement. n An alternative technique is to specify enforced motion using the large mass method.

S17-9 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation USING LARGE MASS METHOD IN FREQUENCY RESPONSE n In frequency response the input and response are assumed to be sinusoidal functions resulting in a simplified dynamic equation of motion:

S17-10 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation USING LARGE MASS METHOD IN FREQUENCY RESPONSE (Cont.) n As in transient response, the force required to move a large mass is but the acceleration is assumed to be a sinusoidal function: Therefore, to impose an acceleration, apply:

S17-11 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation RECOMMENDATIONS IN ENFORCED MOTION n Use automatic enforced motion introduced in MSC.Nastran 2001 if possible. n If using large mass method: u Large mass should be ~10 6 times the structure mass (any higher causes numerical errors, often identified by elastic modes not being found). u Retain rigid body modes for analysis. u Be careful with units - many times enforced acceleration is specified in terms of g (acceleration constant) rather than in direct units (such as in/sec 2 ). u Use a small model to verify solution procedure.

S17-12 NAS122, Section 17, August 2005 Copyright 2005 MSC.Software Corporation